This invention relates generally to phase contrast magnetic resonance imaging (MRI), and more particularly the invention relates to phase contrast MRI using a multicoil phased array for magnetic resonance signal detection.
As described by Pelc et al. ("Phase Contrast Cine Magnetic Resonance Imaging" Magnetic Resonance Quarterly Vol. 7 No. 4, pp. 229-254, 1991), phase contrast MRI refers to a family of MR imaging methods that exploit the fact that spins that move through magnetic field gradients obtain a different phase than static spins, enabling the production of images with controlled sensitivity to flow. Among the advantages of phase contrast MRI are its adjustable sensitivity to flow and, in some cases, the quantitative nature of the resulting data. In the most common method, two data sets are acquired with a different gradient first moment in one direction, and the pixel by pixel phase difference .DELTA..phi. in the resulting images is used to measure the velocity, V, in the direction of the gradient change: .DELTA..phi.=.gamma..DELTA.M.sub.1 V, where .DELTA.M.sub.1 is the change in gradient first moment and .gamma. is the gyromagnetic ratio. Four measurements can be used to measure all components of velocity. Sometimes, for aesthetic reasons related to the very high noise in .DELTA..phi. in regions of low signal, "magnitude weighted" or "magnitude masked" velocity images with pixel intensities proportional to M.DELTA..phi. are produced, where M is the magnitude image.
Phased array multicoils can be used in MRI to improve the signal-to-noise ratio (SNR) of images. See, for example, Roemer et al. "The NMR Phased Array" Magnetic Resonance in Medicine, Vol. 16, pp. 192-225, 1990; Hayes et al. "Noise Correlations in Data Simultaneously Acquired from Multiple Surface Coil Arrays" Magnetic Resonance in Medicine, Vol. 16, pp. 181-191, 1990; and Hayes et al. "Volume Imaging with MR Phased Arrays" Magnetic Resonance in Medicine, Vol. 18, pp. 309-319, 1991. Basically, the multicoils allow large field of view imaging with the SNR of a small surface coil. Separate images are acquired and reconstructed from each of the elements in the multicoil array. These separate images are then combined into a single image, with each coil dominating the spatial regions where its SNR is the highest. Although various algorithms for this combination process have been proposed, the most commonly used methods use the magnitude of the pixel values in the individual images. For example, one very common method forms images as the square root of the sum of squares of the individual images.
The combination of phase contrast techniques and phased array coils would be very desirable in order to improve the SNR of the velocity-sensitive images. However, a simple and efficient method for combining the information received from the various coils that retains a proportionality with velocity is needed.